The Central Valley Water Reclamation Facility (CVWRF) in Salt Lake City, Utah has been operating a biosolids composting operation since 1992. About 30 wet tons/day at 17 percent solids — about one-third of CVWRF’s total daily production — were windrow composted at a remote site adjacent to the Salt Lake City landfill. The remaining biosolids were, and continue to be, land applied on rangeland to grow forage for cattle.
“Composting was done in windrows with five turns in the first 15 days to meet time and temperature pathogen and vector attraction reduction requirements,” recalls Thomas Holstrom, Process Engineer and Assistant General Manager of CVWRF. “The biosolids were composted remotely so that odors wouldn’t be a problem.”
Eventually, a landfill developer wanted to use the property, which had much higher value as a landfill than as a composting site. Permitting a new 39-acre site, adjacent to the landfill was a challenge because the property was located in a different jurisdiction than the existing composting site (Salt Lake County vs. the city). As a result, CVWRF decided to move the composting operation to property adjacent to the wastewater treatment plant. The property, along with an adjacent golf course and driving range, is owned by CVWRF and serves as a current buffer and future expansion site.
“Odor and process control capability was going to be critical, and we knew we would have to compost on a much smaller footprint,” says Holstrom. “We were considering our options and were approached by John Bouey of Managed Organic Recycling, Inc. (MOR), which makes equipment for the covered aerated static pile (ASP) composting system. In 2006, MOR built a pilot-scale ASP system for CVWRF to evaluate.” Eventually, a 3.5-acre composting pad was constructed in 2010 and is adjacent to the driving range. Full-scale operations — using the MOR composting system — began in February 2011.
The CVWRF has two egg-shaped anaerobic digesters and five 85-foot cylindrical digesters with floating covers. Hydraulic retention time is 42 days, with about 62 percent volatile solids destruction. After digestion, the solids are run through a belt filter press and then held in large storage silos. Green waste is used as amendment. Originally, CVWRF only used ground chips brought to the site by tree trimmers. “Those chips average about 1.5-inches in size, and we found we needed bigger chips to improve porosity of the mix,” says Pete DeLigt, compost operations manager. “We purchased a Peterson 710 horizontal grinder and grind green waste into about 2-inch sized chips.” The facility continues to receive material from tree trimmers, which it also uses. It needs a total of 100 cubic yards/day of amendment.
A trailer-mounted Roto-Mix unit (35 cy capacity) is used to blend the biosolids and wood chips. An operator puts wood chips into the mixer, which is pulled into the storage building to add the biosolids (the mix is 3:1 wood chips:biosolids by volume; 1:1 by weight). A bed of wood chips is laid down over aeration trenches on the composting pad, and the mixer unloads directly onto the pad via conveyor discharge. Initial moisture content is 65 percent. “Originally, we would turn the windrow before covering it, but the operators got so good at backing the mixer up to the pad and unloading it that we decided to forgo that step,” says Holstrom. “In addition, turning the windrow before covering it became an odor source, especially during the summer.”
Each windrow is 160-feet long, 25-feet wide and about 10-feet tall. (The expanded Polytetrafluoroethylene (ePTFE) breathable micropore compost covers, a membrane between two layers of polyester fabric, are 38.5-feet wide by 175-feet long.) The MOR system, purchased by CVWRF, includes 14 covers, a unit to place and remove the covers, computer-controlled 5 HP variable speed blowers and Reotemp temperature probes. The AirFloor aeration system, supplied by BuildWorks, provides 100 percent coverage of each 160-foot long windrow. “We can make the cover with different pore sizes to increase or decrease the breathability of the fabric to match the oxygen uptake rates of the microbes,” explains Bouey.
The ASP composting process takes place in two phases. During the active composting phase, about a 4-week period, temperatures in the piles reach over 1500F, which are maintained for at least three consecutive days to meet the USEPA Part 503 Processes to Further Reduce Pathogens. The next phase (temperatures below 1310F) is curing, and lasts generally two weeks. The windrows are uncovered and turned twice a week, during curing. The CVWRF purchased a SCARAB windrow turner with a tunnel width of 27-feet at the base. “We wanted to process as much as we could on a small footprint, which is why we purchased the biggest unit that SCARAB made,” says DeLigt.
Initially, the piles require about 1,000 cubic feet/hour (cfh) of aeration/dry ton of compostable material, but as the pile matures oxygen demand drops to below 500 cfh/dry ton, notes Bouey. He adds that the MOR ePTFE compost covers meet California’s South Coast Air Quality Management District’s Rule 1133.2 for over 80 percent reduction in volatile organic compounds (VOCs). They were also tested at CVWRF for odor emissions. Although the anaerobically digested biosolids used in the tests at the original composting site emitted high levels of ammonia (over 500 mg/m3 even after the 30 day active composting cycle), he says that dilution to threshold levels were reduced by over 80 percent.
So far this winter, CVWRF has not experienced any challenges with removing the covers due to freezing. “Our weather extremes here are fairly temporary,” says Holstrom. “It ends up getting above freezing frequently enough to remove the covers.” He adds that the blowers are coming on for 3 minutes every 20 minutes, whereas in the warmer weather, they came on for 3 minutes every 12 minutes. A higher aeration rate is necessary in the summer months to control temperatures that can reach over 1600F.
The piles are monitored over a 200,000 sq.ft. area with 26 wireless Reotemp temperature probes (EcoProbes). Data from the probes is transmitted back through a receiver antenna to a wall-mounted receiver box, and is funneled into a control system that operates the blowers and displays the temperatures, and tracks individual batches. Data is accessible remotely.
The learning curve for CVWRF staff was pretty steep in terms of knowing how to optimize the composting process and control odors. “We are evaluating whether to remove the cover at 4 weeks, turn the pile and recover it for 2 weeks, or else keep it covered for 6 weeks and then treat it as a windrow,” says DeLigt. “Our testing is ongoing.” Adds Holstrom: “We had trials and tribulations, but we are now getting to the point where the trials have subsided and we are making a very nice compost.”
Because it is using larger wood chips, CVWRF decided to purchase a McCloskey trommel screen, which is being delivered in February. “We’ve been using a small USA Starscreen that we bought 2 years ago,” says Holstrom, noting that the facility needed a machine with more throughput capacity.
The next frontier for CVWRF is increasing sales of its Oquirrh Mountain Compost products, he adds. “The first challenge was the new composting process. Now it is selling compost. We’ve been sold out in previous years, but this past year, we had a very wet spring. That, combined with the slow economy, and we have a fair amount of stockpiled product.”
The majority of compost, which is USCC STA (Seal of Testing Assurance) certified, is sold for $18/cy to landscapers. Homeowners also come to the plant to purchase compost; CVWRF will deliver loads ranging in volume from 6 cy (minimum) to 30 cy. “And we just purchased a small Rotochopper bagging machine that will produce 4 to 10 bags/minute,” says Holstrom. “I issued a purchase order to buy the first 5,000 bags!” The bagged product will meet the small gardener market segment as well as create a promotional and sales tool for giveaway at local trade shows. It’s anticipated that the 1.5 cu.ft. bags will sell for about $4/bag to be competitive with similar products on the market.
Marketing includes having booths at home and garden shows, landscaper events and various community festivals. “A lot of people don’t understand the benefits of compost, so education and outreach is important,” says DeLigt. “We do have to deal with traditional perceptions about biosolids, but a lot of our customers swear by our compost. One step we will be taking is to turn the windrows more often during curing to try to reduce the slight odor that is associated with the product.”
The CVWRF is looking into making product blends. “Soils in Utah are traditionally not very good,” says Holstrom. “Soil amendments are a market we would like to pursue. Our long term goal is to compost all of the biosolids CVWRF produces, but that will be driven by product sales.”